The objective of this proposal is to perform an analysis of a flat type revertant cell line derived from a Ki-MSV transformed non-producer mouse cell line, in relation to the following factors which affect the expression of the viral genome: a) Integration site, orientation and representation of the viral genome DNA in host cell DNA; b) Initiation site, and processing of viral transcripts from viral and/or host DNA; c) possible identification of the Ki-MSV mRNA which codes for protein(s) responsible for cellular transformation. The methods employed are: 1) preparation of specific probes which detect specific loci of the Ki-MSV genome; 2) reassociation kinetics to quantitate virus-related sequences; and 3) restriction enzymes and the filter transfer technique to analyze viral DNA in relation to host DNA. The information obtained by this approach will lead to a better understanding of the mechanisms of cellular and viral control of transformation.